Our arrangement archive has links to each of the programs/plugins that are required to view the near the top of the page.  Here's an overview:

Finale (.mus) Files require the Finale Notepad, which can be downloaded for free from www.codamusic.com.

Sibelius (.sib) Files require the Scorch plugin, which can be downloaded for free from www.sibelius.com/scorch.

PDF (.pdf) Files require the Adobe Reader, which can be downloaded for free from www.adobe.com/products/acrobat/readstep2.html.

Most other files should open with software you already have on your computer.